RFC: C2: Anti-dependence on a load with a control in presence of a membar

[Vladimir Ivanov]

Hi,

I'm seeing unschedulable graph being produced during GCM when adding 
anti-dependence to a load node with a control dependency. I found the 
root cause, but can't decide how to fix it.

Here are steps which lead to the broken graph:

  (1) The load causing problems (#391) is added as part of specializing 
ArrayCopy for small arrays (added as part of JDK-6912521 [1] in 9). Both 
control & memory are tied to AllocateArray. (IR [2])

  (2) EA proves that AllocateArray (#363, destination) is scalar 
replaceable and during split_unique_types() updates corresponding 
MemoryMerge (#379) and it allows to directly use memory produced by 
ArrayCopy (#255, source) bypassing the allocation & membar (#348). (IR [3])

  (3) After allocation elimination, the load control dependency is 
switched to MemBarCPUOrder (#348) which was immediate dominator of 
eliminated allocation (IR [4])

  (4) After matching the load has control on the membar, but not memory 
(IR before [5] and after [6] matching.)

  (5) During GCM, anti-dependence from membar (#317) to the load is 
added, but it makes the graph unschedulable which then triggers the 
assertion [7] during LCM.

Relevant places in the code: [8]

Everything looks fine, except updates of MergeMems in step #2:

   * the load is pinned to the proper branch after deciding what 
direction to go;

   * wide membars do need anti-dependences on loads

So, as a fix I'd disable memory edge updates which bypass any membars. 
Does it sound reasonable or am I missing something important?

Best regards,
Vladimir Ivanov

[1] https://bugs.openjdk.java.net/browse/JDK-6912521

[2] http://cr.openjdk.java.net/~vlivanov/misc/antidep/01_initial.png

[3] 
http://cr.openjdk.java.net/~vlivanov/misc/antidep/02_ea_split_unique_types.png

[4] 
http://cr.openjdk.java.net/~vlivanov/misc/antidep/03_after_alloc_elimination.png

[5] http://cr.openjdk.java.net/~vlivanov/misc/antidep/04_before_matching.png

[6] http://cr.openjdk.java.net/~vlivanov/misc/antidep/05_after_matching.png

[7]
#  Internal Error 
(/Users/vlivanov/ws/jdk/panama-dev/open/src/hotspot/share/opto/lcm.cpp:1169), 
pid=90414, tid=14851
#  assert(false) failed: graph should be schedulable


[8] http://cr.openjdk.java.net/~vlivanov/misc/antidep/webrev/